Cisco CCENT 640-822 Certification Exam Training :

What Are Broadcasts And Broadcast Storms?

By Chris Bryant, CCIE #12933

To pass the Cisco CCENT certification exam, you must not only know what a broadcast is, but why network admins are so concerned about controlling them. As future CCENTs and network admins, you also need to know which devices will help to control broadcasts and which ones will not! In this tutorial, we’ll discuss exactly what broadcasts are and why we have to be wary of them on our network.

When a host device (generally a PC) on a network sends a message, that message is either a unicast, a multicast, or a broadcast. These message types are much like the three message types you and I send when we speak!

If you and I are having a private conversation, what I’m saying is destined only for you and what you’re saying is destined only for me. The network equivalent of that is a unicast – a message with one and only one intended destination.

If you were speaking to a group in a room, your voice is sending a message destined only for that group in that room. The network equivalent of that is a multicast – a message that is intended for members of a certain group, and that no one else should receive.

But if you go outside and start screaming your message, you’re sending the equivalent of a broadcast, a message that is intended for everyone! In the real world, this could lead to a very sore throat, and it could also cause you some trouble with the law. Why? Because not everyone wants to hear what you have to say, and one of them just might call the cops!

Broadcasts on a network won’t result in a call to the police, but they can still cause trouble for much the same reason. Just because one host sends a message, it doesn’t meant that every other host on the network needs to hear that message. It also doesn’t meant that we as network admins want every other host to receive that message, because the receiving host and some other networking devices on the network still have to process that message.

That may not sound like a problem, and if there are only a few broadcasts on your network, it may not be. The problem comes in as more and more broadcasts are generated. Broadcasts tend to result in more broadcasts being generated by network devices, and as the broadcasts continue to multiply, they may snowball into a broadcast storm. Broadcast storms occur when there are so many broadcasts on a network that it cripples the network’s ability to operate normally.

Today’s networks don’t carry as many broadcasts as they once did; most of today’s popular routing protocols use multicasts to operate rather than broadcasts. It’s still a good idea to know how to limit broadcasts, though, both for your CCENT studies and working with real-world networks. Let's take a look at some popular methods of limiting broadcasts in the next installment of this Cisco CCENT / CCNA certification exam tutorial series!

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To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com